The invention uses various materials which are electrically either conductive, insulating or semiconducting, although the completed semiconductor circuit device itself is usually referred to as a "semiconductor". One of the materials used is silicon, which is used as either single crystal silicon or as polycrystalline silicon material, referred to as polysilicon or "poly" in this disclosure.
An electrically programmable and electrically erasable programmable access memory (EEPROM) utilizes floating gate transistors which are programmed and deprogramed by Fowler-Nordheim tunneling through a thin gate oxide window near the drain side of the transistor. The transistor is an n-channel transistor and programming is achieved by applying a relatively high positive potential (voltage or EMF) to a control gate. A floating gate is isolated from the control gate and the source and drain of the transistor and is interspaced between the control gate and the transistor substrate. The floating gate has a coupling coefficient of between 0 (no isolation from the substrate) and 1 (no isolation from the gate).
The transistor may be programmed by applying a high positive potential to the control gate with respect to the drain. The floating gate reaches a potential proportional to the control gate potential times the coupling coefficient. If the potential of the control gate is sufficiently high, Fowler-Nordheim tunneling through a thin gate oxide window near the drain side of the transistor closes the floating gate to become negatively charged. This results in threshold voltage of a higher value, usually sufficient to exceed the potentials at which the transistor normally operates, thereby deprogramming the transistor. Deprogramming is accomplished by applying, from the gate, a high negative potential to the drain side of the transistor.
The floating gate technique works very well n-channel transistors, but is difficult to achieve in the case of p-channel transistors. This is because of the oxide-silicon barrier for holes is much greater than that of electrons. For this reason, prior art programmability of inverter devices has been difficult.